The present invention relates to foundry practice and in particular, a way of casting hardenable iron articles to precise dimensions in sand molds. The invention pertains more particularly to foundry practice as employed for casting hardenable iron for making bodies for engine tappets of the type having a roller follower provided on one end of the tappet for contacting a rotating cam shaft. Hardenable iron bodies for roller-follower tappets are required to be cast with a pocket or socket formed in one end thereof for receiving the roller-follower partially therein. The wall of the end pocket is then cross bored for the roller-follower axle.
When it is desired to convert tappets for an existing production engine having a flat hardened face for sliding friction contact with the cam shaft lobes, to roller-follower configuration, it has been found difficult to form the necessary pocket in the end of the tappet for receiving the roller-follower and yet provide sufficient material strength in the wall surrounding the pocket for supporting the roller-follower axle. In order to maximize the size of the roller-follower for a given size or diameter tappet, the wall of the pocket formed in the end of the tappet for receiving the roller-follower has remained sufficiently thin that stress cracking has occurred in the portions of the walls supporting the roller-follower axle. Accordingly, it has therefore been necessary to tightly control the tolerances on the thickness of the wall surrounding the pocket, which has in turn, required close tolerance control and accurate positioning of the correspondingly shaped form in the sand mold in order to maintain the desired tolerances on the wall of the pocket in the cast article.
Where the hardenable iron tappet body is cast in sand molds, it has been found difficult to accurately control the position of the form or pattern in the sand mold to create the pocket in the end of the tappet. It has also been found difficult to maintain the tolerances on the portion of the sand mold forming the pocket. This has been found to be particularly difficult where the tolerances on the pocket dimension of the as-cast article cannot exceed .+-.0.010 inch (.+-.0.259 mm). The problem of locating the tappet pocket forming in a sand mold has been particularly troublesome where a sand mold is desired having a plurality of mold cavities for simultaneously pouring several tappet bodies in one sand mold.
Thus, it has been desired to find a way or means of forming a precision mold cavity in a sand mold and particularly for casting the roller-follower receiving pocket in a hardenable iron tappet body.